The present invention relates to an image reading apparatus for obtaining image data from a reading object for example and an image forming apparatus having its image reading apparatus.
An image scanner obtains image data by illuminating, with light, a reading object such as a sheet-like document, a book or a solid material and causing obtained reflected light to be photoelectrically converted by an image reading sensor.
The image reading apparatus comprises a document retaining section (document table) for retaining the reading object, an illumination unit including an illumination lamp and configured to illuminate, with the lamp, the reading object set on the document table, a line charge-coupled device (CCD) sensor serving as an image reading sensor, an optical set (a plurality of mirrors and image formation lens) provided between the document table and the CCD sensor to allow reflected light from the reading object to be image-formed on the CCD sensor, etc.
In the above-mentioned image reading apparatus, there are sometimes the cases where the temperature within the apparatus is cooled, for example, in a earlier hour such as in the morning and a belt for transmitting the rotation of the motor as well as a wire for moving the illumination unit is hardened, for example, in the winter and in the high latitude regions. Immediately after the power source is turned ON, therefore, a load on the motor is augmented in comparison with those other than the lower temperature environments (for example, an ordinary temperature environments).
If the motor is to be rotated as usual with the load augmented, the torque involved is relatively lowered so that the motor cannot be rotated smoothly.
When the illumination unit is moved with the torque in an insufficient state, there occurs a problem that a read-out image will be distorted due to a vibration of an illumination unit. In a back scanning for returning the illumination unit back to a standby position, there also occurs the problem that the illumination lamp, structure, etc., will be destroyed upon the impact of the illumination unit against the frame of the image reading apparatus.
In order to solve a relative, insufficient motor torque, a proposal has been made, for example, in Jpn. Pat. Appln. KOKAI Publication No. 5-100324 by which a motor drive current supplied at a lower temperature time (under the lower temperature environment) is more increased than usual (under the ordinary temperature environment).
It is suggested that, in Jpn. Pat. Appln. KOKAI Publication No. 5-38193, the circumferential temperature is detected and, based on the temperature, the level of the motor drive current is returned back to a normal state.
The level of the motor drive current is set to include an allowance even at a normal operation time so that it may be less affected by a variation in power supply voltage and an unexpected disturbance.
If the level of the motor drive current supplied to the motor is set to include any excessive allowance of an extent not required even at other than a lower temperature environment, the motor is sometimes vibrated. The vibration generated in the motor is transmitted by a belt or wire to the illumination unit and there occurs a problem that any read-out image is distorted. It is to be noted that, when the current supplied to the motor and motor drive circuit becomes greater than a predetermined level, the noise is increased each time the current is supplied. Further, the motor drive current including any excessive allowance simply increases dissipation power.